1. Field of the Invention
The present invention relates to apparatus and methods for analyzing particles in a fluid and, more particularly, to apparatus and methods for counting and analyzing fine particles such as blood cells in a fluid such as blood through detection of a change in electrical and optical characteristics of the fluid.
2. Description of the Related Art
Apparatus has been known such as a Flow Cytometer as disclosed in the specification of U.S. Pat. No. 4,263,508, in which a specimen liquid suspending fine particles flowing through a restricted passage of very small cross-section in the center of a flow cell is irradiated with a laser beam so as that the nature, construction and other properties of the particles are known from, for example, information available from the signal pulses of the scattered light or fluorescent light from the particles, e.g., peak values of the pulses or integrated values of the pulses. The specification of U.S. Pat. No. 2,656,508 also discloses a counting apparatus for counting blood cells in blood. In this apparatus, the sizes of various kinds of blood cells are measured and the quantity of the blood cells are ascertained through measurement of changes in the electrical impedance of the blood, particularly through measurement of the peak level of signal pulses generated when the blood cells suspended in a suspension liquid pass through a fine passage while being irradiated from a laser beam.
The signals obtained when the specimen particles move across a laser beam or through a minute passage are in the form of electrical pulses. In the known analyzing apparatus, the analysis is conducted by measuring the peak level or the integrated value of these electrical pulse signals.
In these known particle analyzing apparatus, the signal pulses are directly input to peak detection circuit or to an integration circuit and the peak detection circuit or the integration circuit is directly operated in accordance with timing pulses which are produced as a result of comparison between the signal pulses with a predetermined reference level. More specifically, the pulse signal is picked up only when the level thereof exceeds a reference level, and only then is it used to charge capacitors in the peak detection circuit or the integration circuit. In this method, therefore, the period of capacitor charge is shorter than the period corresponding to the actual width of the signal pulses, so that the resulting integration value inevitably involves error in that the value becomes smaller than the value which corresponds to the integration over the actual pulse width. The peak detection circuit also encounters a problem in that the response of this circuit tends to be delayed particularly when the width of the signal pulse is extremely small or when the level of the peak is very small, with the result that a considerable error is involved in the peak level as measured.